By Brad Brooks and Gabriella Borter UVALDE, Texas (Reuters) – Investigators in Texas were seeking to determine on Saturday how critical mistakes were made in the response to the deadly Uvalde shooting, including why nearly 20 police officers remained outside a grade school classroom as children placed panicked 911 calls for help. Why the officers waited in the hallway nearly an hour before entering and fatally shooting the gunman is at the heart of an ongoing probe by the Texas Department of Public Safety into the massacre of 19 children and two teachers in the deadliest U.S. school shooting i…